The photodissociation of the SiO molecule, which is of considerable astrophysical interest, is studied within the framework of the Floquet theory. Only the one-photon transition process between the two lowest (1) Sigma(+) states is considered. The computation of the electronic energy surfaces is performed with the MOLPRO code. The wave operator theory is used to select the active space related to this process, and to build the Floquet resonance states which constitute this active space. Particular attention is paid to the role of the shape resonance states induced by the presence of a barrier on the E (1) Sigma(+) surface.